1. Field of the Invention
The present invention relates a protection circuit device using MOSFET and method of manufacturing the same, particularly enabling to build in a secondary battery and carrying out battery management.
2. Description of the Related Art
As the spread of pocketable terminal, small lithium ion battery having large capacity is desired. A protection circuit device carrying out battery management of charge-discharge of the lithium ion battery must be smaller and sufficiently resist to short of load. As such protection circuit device is built in vessel of lithium ion battery, miniaturization is required, and freely using COB (Chip on Board) technique using many chip parts meets require of miniaturization. However on the other hand, as switching element is connected to lithium ion battery in series, it is needed to make ON-state resistance of the switching element extremely small. This is indispensable factor to lengthen talking time or stand-by time in pocket telephone.
A protection circuit carrying out concrete battery management is shown in FIG. 19. Two power MOSFETs Q1 and Q2 are connected to a lithium ion battery LiB in series to detect voltage of the lithium ion battery LiB with a control IC. The detection carries out on-off control of the two power MOSFETs Q1 and Q2, and the lithium ion battery LiB is protected from over-charge, over-discharge, or load short. The power MOSFETs Q1 and Q2 connect drain electrodes D in common, source electrodes S are arranged respectively at both ends thereof, and gate electrode G of each MOSFETs is connected to the control IC.
At charge, a power source is connected to ends of the circuit, and charge current is applied to the lithium ion battery LiB to arrow direction so as to charge. When the lithium ion battery LiB becomes over-charge state, voltage is detected by the control IC, gate voltage of the power MOSFET Q2 becomes L (low level) from H (high level), and the power MOSFET Q2 becomes off and cuts the circuit so as to protect the lithium ion battery LiB.
At discharge, both ends of the circuit are connected to a load and operation of a pocketable terminal till designated voltage. However when the lithium ion battery LiB becomes over-discharge state, voltage is detected by the control IC, gate voltage of the power MOSFET Q1 becomes L from H, and the power MOSFET Q1 becomes off and cuts the circuit so as to protect the lithium ion battery LiB.
Moreover at load shot or over-current, much current flows the power MOSFETs Q1 and Q2 so that voltage of both ends of the power MOSFETs Q1 and Q2 suddenly rises. Because of that, the voltage is detected by the control IC, and similarly at discharge the power MOSFET Q1 becomes off and cuts the circuit so as to protect the lithium ion battery LiB. However as large current flows at short time till the protection circuit operates, it is required that peak drain rash current flows much to the power MOSFETs Q1 and Q2.
As two N-channel type power MOSFETs Q1 and Q2 are connected to the lithium ion battery LiB in series in the protection circuit, low-ON-state resistance (RDS(on)) of power MOSFETs Q1 and Q2 is most required point. Therefore development raising sell density by fine pitch machining at manufacturing the chip is driven forward.
In detail, although sell density was 7.4 million per a square inch and ON-state resistance was 17 mΩ in planer structure that a channel is formed on a semiconductor substrate surface, at first generation of trench structure forming a channel at side face of trench, sell density is extremely improved in 25 million per a square inch and ON-state resistance is decreased in 27 mΩ. Further in second generation of the trench structure, sell density is 72 million per a square inch and ON-state resistance is decreased in 12 mΩ. However making fine has limit, and there is a limit to decrease ON-state resistance extremely.
FIG. 20 is a plan view describing a protection circuit device mounting such a power MOSFET improved in sell density. Although circuit parts shown in FIG. 19 are mounted actually, the parts are not shown all in the figure. A conductive path 2 comprising copper foil is formed on both face of an insulating board 1, and has multilayer interconnection where the conductive paths 2 of upper face and lower face of the board are connected through through-hole (not shown) at desired position. Power MOSFETs 3 and 4 are resin-molded in an external form of SOP8 for surface mounting, two terminals 5 and 5 connected to drain electrodes go out at one side, and at the opposite side, a gate terminal 7 connected to a gate electrode and a source terminal 8 connected to a source electrode go out. Symbol 9 is a control IC, symbol 10 are chip capacitors corresponding to C1 to C3 of FIG. 19, and symbol 11 are chip resisters corresponding to R1 and R2 of FIG. 19. Symbols 12 and 13 are external terminals corresponding to LP2 and LP3 of FIG. 19. The external terminals are fixed on pads 14 formed at part of the conductive path 2 by solder. Although the protection circuit device is formed in a suitable shape to put in the case of the lithium ion battery, miniaturization is the largest problem for fundamental needs.
FIG. 21 shows a section structure of the power MOSFETs 3 and 4. A frame is a pressed frame comprising NK-202 (copper 97.6%, tin 2%) as material, and a bare chip 23 of the power MOSFET is fixed with a preform material 22 comprising solder or silver paste on a header 21 of the frame. A drain electrode is formed by gold lining electrode (not shown) on lower face of the bare chip 23 of the power MOSFET, and on upper face, a gate electrode and a drain electrode are formed by deposition of Aluminum. As connected to a header 21, a drain terminal of the frame is connected to the drain electrode directly, and the gate electrode and the source electrode are electrically connected to a gate terminal 7 and a source terminal 8 by ball bonding using gold bonding wire 24. Therefore ON-state resistance of power MOSFET is influenced ON-state resistance existing in frame material, preform material, material for the bonding wire, and electrode material of the source electrode on the upper face of the chip for decreasing ON-state resistance.
FIG. 22 and FIG. 23 are plan view describing the prior art decreasing ON-state resistance devising bonding wire. FIG. 22 is a view where current capacity is improved by increasing the bonding wire 24 connecting the source electrode and the source electrode 8 to four wires. Further FIG. 22 is a view where current capacity is improved by increasing the bonding wire 24 connecting the source electrode and the source electrode 8 to four wires, two short wires and long two wires, and where resistance of the source electrode is decreased by broadening bonding portion to the source electrode.
Difference of ON-state resistance depending on the conventional mounting structure of power MOSFET is completed in FIG. 18. Sample A and Sample B are the conventional mold structure of SOP8, Sample A corresponds to the structure of FIG. 22, and Sample B corresponds to the structure of FIG. 23. These figures show, in the case that bonding wires is changed to combination of two short wires and two long wires from four short wires, decrease of ON-state resistance of 1.33 mΩ, from 13.43 mΩ to 12.10 mΩ, however changing solder to Ag paste can not decrease ON-state resistance.
However a present state is that small, light weight pocketable terminal and long life of built-in battery thereof are more strongly required. In the sate, there is a problem that useful solving means breaking down mounting structure of protection circuit device using power MOSFET, realizing low ON-state resistance, and realizing small protection circuit device using MOSFET is not found out.